In general, an ultra-wideband (UWB) antenna refers to a communication system with its fractional bandwidth larger than 25%, or greater than 1.5 GHz. Since an UWB antenna technology involves carrier-free, low power consumption and high-frequency digital pulses for data transmission, the required transmission bandwidth tends to be pretty large. The current UWB technology is mainly used for public safety and broadband wireless communications. In United States, as of February 2002, the Federal Communications Commission (FCC) has released UWB for equipments, such as ground penetrating radar systems, through wall imaging systems, and medical imaging systems for the purpose of public safety utilizations. For broadband wireless indoor communications, FCC also approved the frequency range of 3.1-10.6 GHz for UWB communication and measurement systems. The Taiwan Telecommunication has also included this spectrum of frequencies for the future utilization plan.
In the teams of academics and industries, researches on the UWB antenna are mostly based on wideband matching, or multiple-resonance-path perspectives. In terms of packaging types, UWB antennas are mostly in shape of monopole or dipole variations.
U.S. patent publication 2005/005,232,2A1 disclosed an antenna suitable for UWB communication systems. Referring to FIG. 1, a patch radiation element 101 that is smaller than a dielectric substrate 108, is formed on the surface of the substrate 108. The radiation energy of the patch radiation element 101 is activated by the current fed via the feeding line 103. Wherein, the bandwidth of the antenna is controlled by air gap slot 102 formed within the patch radiation element 101. To accomplish the impedance matching between the radiation element 101 and the feeding line 103, there are matching elements 104 and 105 formed between the radiation element 101 and the feeding line 103.
Compared with an ordinary monopole antenna, this type of antenna design advantages itself as providing broad enough impedance bandwidth, which can meet the general need for UWB applications. This type of antenna, however, has a high profile of 30×35 mm2 in dimension, which is hard to be applied to small size personal communication equipments, such as mobile phones, personal digital assistants, etc.